Cryptosporidium species are among the world's most ubiquitous diarrhea-causing pathogens. Although cryptosporidiosis can be severe and sometimes lethal in AIDS patients, not a single drug is available for the treatment of cryptosporidiosis in this vulnerable population. Immunotherapy is regarded as a promising approach but the immunodeficiency imposed by HIV is such that T helper cells are reduced both in number and function, particularly in the gut. In working to address this therapeutic gap, we discovered that stimulation of innate immune responses with soluble Toxoplasma antigen (STAg) is a rapid and effective treatment for Cryptosporidium parvum infection in IFN? deficient mice. As STAg stimulates innate immunity in a T-cell independent manner, understanding the mechanisms by which STAg displays anti-Cryptosporidium activity would inform development of a novel immunotherapy for cryptosporidiosis in immunocompromised patients. The anti-Cryptosporidium activity of STAg is likely due to the stimulation of innate immune responses because the effect is rapid, reducing oocyst shedding within the first two days of treatment. Importantly, our preliminary data show that STAg treatment is effective in the absence of IFN?. The present studies constitute the first comprehensive analysis of IFN? independent innate immune mechanisms that lead to clearance of Cryptosporidium organisms. Moreover, these studies will be the first to characterize the early host-Cryptosporidium transcriptome in vivo, a data set that will be an invaluable resource to the Cryptosporidium research community. The aims of this study are: Aim #1: To characterize the early immune response required for clearance of Cryptosporidium infection in wild type and IFN? deficient mice. In this aim, we will optimize our model with respect to timing and dosage of STAg treatment, and determine if the effect of STAg can be recapitulated by treatment with purified TgPRF. Aim #2: To identify changes in host and parasite gene expression associated with STAg medicated protection against Cryptosporidium infection. Using RNA seq, we will characterize the transcriptome of ileum tissues obtained from wild type and IFN? deficient mice infected with C. parvum and treated with STAg. Analyses of this comprehensive time course will generate hypotheses on the mechanism of STAg action against C. parvum that will be tested using immune cell depletion or knock-out mice. These studies will identify the key innate immune responses effective against the AIDS opportunistic pathogen Cryptosporidium, and provide data necessary for the development of both IFN? dependent and independent immunotherapeutics for currently untreatable cryptosporidiosis.